Chapter 8

SHELDON, LEONARD, AND LESLIE: THE THREE FACES OF QUANTUM GRAVITY

Andrew Zimmerman Jones

At the beginning of The Big Bang Theory’s second season, a romantic relationship ends abruptly, virtually before it begins. The relationship has the potential for some good laughs, but it fizzles after just one episode. No, I’m not talking about Leonard and Penny’s romance (version 1.0), but instead the second episode of the season, “The Codpiece Topology,” in which a rebounding Leonard begins to date his physicist colleague Leslie Winkle. Recall the heated debate between Leslie and Sheldon about loop quantum gravity and string theory. Leslie believes that loop quantum gravity best describes the universe, and Sheldon disagrees (strongly). Leonard is pulled into the fracas and diplomatically asserts, “Well, there’s a lot of merit to both theories.” Leslie will have none of it; she complains, “Well, I’m glad I found out the truth about you before this went any further.” Leonard promptly responds, “What truth? We’re talking about untested hypotheses. It’s no big deal.” Leslie presses, “Oh, it isn’t, really? Tell me, Leonard, how would we raise the children? . . . I’m sorry. . . . This is a deal breaker.”

Yet what is the “deal breaker,” exactly? Her quarrel was (originally) with Sheldon. Did Leonard fail to defend her honor in some sufficiently chivalrous way? Sure, the apartment is bereft of strumming minstrels, but the relationship ends in a way that leaves many of us a bit perplexed. At the very least, it is more mysterious than the breakup Leonard experienced only one episode earlier in “The Bad Fish Paradigm”: Penny was insecure about her lack of formal education, and Leonard was not sufficiently sensitive to it.

Believe it or not, we can better understand (and appreciate) the Leonard/Leslie breakup if we delve into the rudiments of science and how it is performed in our modern era. This will require encountering the thorny question of science’s ultimate goal and what methods should be employed to reach it.

The Methods of Science

The scientific method involves observing some behavior in nature, formulating a rule that would explain that behavior, and then constructing a test to see whether that rule holds up in a new, but relevantly similar, situation. What, though, is the “most important” part of this process? Is it the coming up with the rule (theoretical science) or the testing of that rule (experimental science)? What if each is about equally important?

Leonard and Howard are practical and hands-on about their scientific work, but Raj seems to be something of a mix between a theoretical and an experimental scientist. He is an astrophysicist who, at least in “The Pirate Solution,” works for (and not with) Sheldon to explore “the string theory implications of gamma rays from dark matter annihilations.” Yet in “The Griffin Equivalency,” Raj is recognized in People magazine for discovering a planet-size object. Sheldon, of course, is firmly (and proudly) on the theoretical side of this divide. In “The Monopolar Expedition,” he is awarded a prestigious grant from the National Science Foundation, but his acceptance requires him to take to the field—to the North Pole, nonetheless. He initially hesitates, explaining, “I’m a theoretical physicist, a career I chose in no small part because it’s indoors.” After accepting the NSF grant, Sheldon quickly displays his lack of practical experimental intuition by having his team practice setting up equipment in the Cheesecake Factory freezer. It takes Leonard only minutes to suggest they could set it up inside and then take their equipment outside.

These fictional scientists and their pending trek to the North Pole illustrate how science tends to unfold. Theoreticians build mathematical models and use them to make predictions that experimentalists then test against the real behavior of physical systems. Today’s theories are often so elaborate that the skill sets involved are fundamentally different in many ways. Even so, we must be wary of the misperception that theoretical physicists alone drive scientific advancement, with the experimentalists along for the ride. The interplay between the theoretician and the experimentalist is much more subtle than that.

One of the key points about scientific ideas, at least according to the philosopher of science Karl Popper (1902–1994), is that they must be falsifiable. In other words, if there’s no experiment that could be conducted (in principle, if not in actual practice) to show an idea to be mistaken, then you can’t really say that the idea is scientific. For example, the idea of God is an idea that, according to most thinkers, can never be falsified and so doesn’t qualify as a scientific idea.

Through this process of falsification, the experimentalists force the theorists to revise their failed theories. Once a theory is falsified, then its status as part of the current “scientific paradigm,” is called into question.1 The current paradigm represents a coherent way to systematize all incoming data; it provides a status quo for how science—and scientists—operate. If a theory fails to account for any new data, this upsets the scientific status quo. Yet theorists don’t immediately abandon their theories wholesale just because of a few contradictions or failed predictions. In the words of philosopher of science Thomas Kuhn (1922–1996), “Normal science . . . often suppresses fundamental [data] novelties because they are necessarily subversive of its basic commitments.”2

That is, scientists go about their business, perfectly happy with the status quo described by their existing paradigm. Occasionally, however, some contradictory or unexpected evidence comes along, disrupting the status quo. Scientists must then make a choice. Some are tempted by the new data, such as a hot new neighbor moving in across the hallway. These scientists are eager to jettison their old paradigm in the hope of breaking new ground, just as Leonard was keen to leave his geeky, lonely self behind to woo Penny. Such scientists are intent on looking for new theories that are “intelligent and beautiful.” Yet just as Sheldon chides that Leonard’s children with Penny will be imaginary, most scientists are not nearly as ready as Leonard to break out on a new path to follow the hot new neighbor—that is, the new, unexpected scientific evidence—wherever it leads. Many prefer to cling desperately to the status quo, refusing to acknowledge that any fundamental change has actually taken place. These scientists prefer attempts to explain the new, seemingly unexpected data under their current paradigm.

This latter strategy coheres with another of Kuhn’s points about science. Regarding the accumulation of contrary evidence, he wrote, “Once it has achieved the status of paradigm, a scientific theory is declared invalid only if an alternate candidate is available to take its place.”3 This suggests that scientists are prone to hold on to a theory longer than they should. Even so, the very nature of science helps ensure that hidden flaws in a theory will be eventually uncovered by someone who is keen to establish a new scientific paradigm. Kuhn also grasps this propensity among scientists, as he wrote, “The scientist must . . . be concerned to understand the world and to extend the precision and scope with which it has been ordered.”4

In “The Alien Parasite Hypothesis,” Sheldon provides a humorous example of a scientist illicitly shying away from the force of countervailing data. He attempts to help his girl-who-is-a-friend Amy Farrah Fowler discover the cause of her irregular biological behaviors in the presence of Penny’s ex-boyfriend Zack. Sheldon’s working under the quasi-paradigm that Amy, like himself, has evolved past tawdry desires of the flesh. They are intellectual beings. Yet all of the data point to the conclusion that Amy is sexually aroused by Zack. Sheldon refuses to accept this, proffering instead the hypothesis that Amy has been infected by an alien parasite, creating biological reactions consistent with sexual arousal. This allows Sheldon to retain his paradigm, but because there is no evidence that Amy has encountered extraterrestrials, his preferred hypothesis also requires him to unduly twist the data. In the end, Sheldon is being unscientific.

Accordingly, consider another fundamental component of science: its skeptical and inquisitive nature. Science ultimately seeks answers wherever they may take us. Nobel Prize–winning physicist Richard Feynman (1918–1988) tersely explained this aspect of science:

Science is a way to teach how something gets to be known, what is not known, to what extent things are known (for nothing is known absolutely), how to handle doubt and uncertainty, what the rules of evidence are, how to think about things so that judgments can be made, how to distinguish truth from fraud, and from show.5

From Feynman’s viewpoint, although scientists are certainly not looking to refute the existing paradigm at every turn, they are also not running from contradictory evidence. Or, at least they shouldn’t be.

Feynman more succinctly described science as “the organized skepticism in the reliability of expert opinion.”6 Again, Dr. Sheldon Cooper has difficulty embracing this sentiment, as evinced in the following exchange from “The Work Song Nanocluster”:

Sheldon: I’m a physicist. I have a working knowledge of the entire universe and everything it contains.

Penny: [unimpressed] Who’s Radiohead?

Sheldon: I have a working knowledge of the important things in the universe.

Sheldon doesn’t know the answer to Penny’s question, but instead of literally running away from the data as he did by quickly leaving Amy’s office, here he simply fails to acknowledge the question. He responds not by attempting to expand his knowledge, but by staying comfortably within his existing paradigm. All that he needs to know is included in his current understanding of theoretical physics. If it appears that there might be any phenomenon that falls outside of it, he dismisses it as unimportant and not worth knowing or even exploring.

A vivid example of scientists following Popper’s and Feynman’s guidance is the discovery of “dark energy.” In 1998, astrophysicists discovered that the expansion of the universe was accelerating. This discovery was unexpected, but the evidence was solid, so scientists modified their models. This is Popper’s falsification at work, because the evidence clearly contradicted the idea of a universe with expansion slowing down or constant (the two prevailing thoughts prior to 1998). It’s one of the clearest cases of experiments leading to new theoretical work. More than a decade later, physicists are still trying to figure out the exact nature of the “dark” (unseen) energy powering this acceleration, as well as its implications for the rest of physics . . . especially since evidence now suggests that about 75 percent of our universe is made up of this unobservable dark energy!

Theoretical “Discoveries”

Physicists take physical properties of the universe and translate them into equations, which they can then use to predict or interpret other physical properties of the universe. For example, consider the current paradigm in physics, sometimes called “modern physics.” This paradigm consists of Einstein’s general relativity, which describes gravity, and a comprehensive model of everything else in nature, called the “standard model” of quantum physics. These two domains both grew out of experimental evidence that did not match up with the classical Newtonian physics, which was dominant for the three centuries prior to Einstein. Today, general relativity and quantum physics are the foundation for all of our understanding of the universe’s workings.

Both general relativity and quantum physics take the form of mathematical equations. In fact, when physicists do any sort of science, they are typically working with equations. Even the experiments, really, are just a means to get the right numbers to plug into the equations. The variables in these equations represent the physical properties of the universe. Experiments set some of these values, but theoretical physicists often alter some parameters to see what results they get, in sort of a “What if the universe were like this?” scenario.

In fact, much of the theoretical work of physics is figuring out the correct equations to use to describe a given situation. These equations have to conform to the known experimental evidence, but the mathematical work can also constitute a sort of new discovery in itself. Consider the discovery of antimatter. Physicist Paul Dirac (1902–1984) was working with equations to describe the behavior of electrons and discovered that they allowed for electrons to be negatively or positively charged. This perplexed him because electrons had been observed only with a negative electrical charge. Because his equations seemed correct, Dirac was led to consider the existence of positively charged electrons: positrons. Did the universe contain “antimatter”—stuff exactly like ordinary matter but with an opposite charge? Indeed. Dirac’s supposition was experimentally confirmed only a few years later by Carl David Anderson (1905–1991).7

It’s easy to imagine this as a discovery that could have been triggered by experiment, instead of by theory. It’s conceivable that physicists would have never considered antimatter until Anderson accidentally stumbled on these strange, positively charged electrons in his experimental work, thus forcing the theoreticians to revise their equations accordingly. Yet it’s unclear whether Anderson would have found positrons had it not been for Dirac’s equations. In any event, it does seem that Dirac’s theoretical work led to the prediction before the experimentalists got to it.

The discovery of antimatter gives insight into the science that seemingly drives “The Monopolar Expedition.” Sheldon seeks to confirm his theoretical work by experimentally detecting slow-moving monopoles at the magnetic North Pole. Monopoles are theoretical but not purely fictional. Theoretical physics equations have predicted that in the high energy levels of the very early universe, there would have been free-standing magnetic poles, called magnetic monopoles. They can be understood by contrasting them with a common magnet (including the Earth itself). Magnets have negatively and positively charged poles (“north pole” and “south pole”), but the problem is that these are always connected. If you take a magnet (or anything else that produces a magnetic field) and cut it in half, you don’t end up with a north pole and a south pole separately; you end up with two smaller magnets, each of which has a north and a south pole. Monopoles, then, are tiny particles that consist of only a single north pole without any connected south pole (or vice versa).

All of Sheldon’s (and actual) current science points to—predicts the existence of—monopoles. The math supports this prediction. If such things existed, there should still be a few of them floating around out there, or they’d come into existence in high-energy collisions in space. Presumably, the Earth’s magnetic field would make them hard to detect normally, which is why a trip to the North Pole is required. If his experiments prove successful—even if it means including Raj, Howard, and Leonard—the ramifications would be great. In his own words: “If I’m able to detect slow-moving magnetic monopoles, I’ll be the scientist who confirmed string theory. People will write books about me. Third graders will create macaroni art dioramas depicting scenes from my life.” Alas, his hopes are dashed; he was not as fortunate as Dirac was about predicting antimatter. He must either revisit his equations or construct a different experiment.

Interestingly enough, Sheldon was correct about his predictions of supersolids in “The Cooper-Hofstadter Polarization.” In this case, Leonard played the role of Carl David Anderson, experimentally verifying Sheldon’s theories. Yet again, Sheldon’s preference for theoretical science takes center stage. He is so certain that the math was correct that he sees no point in presenting their findings at a prestigious conference. Why should he “kow-tow to lesser minds?” Moreover, he forbids Leonard from presenting their findings without him. When Leonard decides to present them anyway, Sheldon attempts to sabotage the presentation (and Howard uploads the whole embarrassing spat to YouTube).

The String Theory Paradigm

Attempts at explaining string theory in the presence of Dr. Sheldon Cooper are bound to provoke ridicule, if Sheldon’s “probing” questions of Dr. Brian Greene in “The Herb Garden Germination” are any indication. . . . “Mua-ha-ha.” Nevertheless, here goes. Recall that current theoretical physics consists of two separate theoretical frameworks. Quantum mechanics describes the ways fundamental particles interact through electromagnetic forces and also through the strong and weak nuclear interactions. There’s a fourth type of interaction, gravity, but quantum theory doesn’t actually cover it. Instead, it is covered by Einstein’s theory of general relativity.

The problem is that predictions made in quantum theory don’t really carry over into general relativity and vice versa. For most things, this isn’t a problem, because the methods of approximation smooth everything over. So physicists can describe most behaviors without running into the conflicts, but they do occasionally creep up in exotic situations, for example, along the edge of a black hole.

Einstein spent the last half of his life trying to create a “unified field theory,” a single equation and paradigm that would encompass all of the fundamental rules about how reality functions. He was unsuccessful at these attempts, though there’s (evidently) some dispute as to why, as tantalizingly conveyed in the following exchange from “The Wildebeest Implementation”:

Sheldon: I must say, ever since you started having regular intercourse, your mind has lost its keen edge. You should reflect on that.

Leonard: Excuse me, but Einstein had a pretty busy sex life.

Sheldon: Yes, and he never unified gravity with the other forces. If he hadn’t been such a hound dog, we’d all have time machines.

(For the record, there is no evidence that Einstein’s promiscuity hampered his work or our owning time machines.)

Eventually, the field of string theory came to prominence as a possible unified theory of quantum gravity. In “The Fuzzy Boots Corollary,” Leonard references this point in passing during his first pseudo-date with Penny:

Penny: So, what’s new in the world of physics?

Leonard: Nothing.

Penny: Really, nothing?

Leonard: Well, with the exception of string theory, not much has happened since the 1930s. And you can’t prove string theory. At best you can say, “Hey, look, my idea has an internal logical consistency.”

Penny: Well, I’m sure things will pick up.

In string theory, all matter is envisioned as tiny vibrating strings, orders of magnitude smaller than the tiniest particle we can now observe. They are so small, in fact, that most scientists who work on the theory don’t believe there’s any way to experimentally observe a string itself, only the consequences of the strings interacting. This, in part, explains Leonard’s comment that “you can’t prove string theory.” He appreciates the math, but the experimentalist in him calls for discernible proof.

Yet there’s more. It’s not just matter that is made up of strings. In quantum mechanics, the forces—electromagnetics, the strong nuclear force, and the weak nuclear force—work only because there are special types of particles, called bosons, that bounce around and make them work. These bosons are also created by vibrating strings, so all matter and its various interactions are described as different types of vibrating strings. This realization led to a particularly amazing prediction that caused many scientists to abandon it. String theory works only if you set up the equations so that the universe has a total of twenty-six dimensions!

This tantalizing issue is raised in the “Pilot” episode, shrouded in the guys’ initial attempt to impress Penny:

Leonard: At least I didn’t have to invent 26 dimensions just to make the math come out.

Sheldon: I didn’t have to invent them. They’re there.

Leonard: In what universe?

Sheldon: In all of them. That’s the point.

Again, we see the clash between the more experimental Leonard and the more theoretical Sheldon. Leonard requires testable predictions. Sheldon believes that the math, carefully done, speaks for itself. In part, this further explains their tiff in “The Cooper-Hofstadter Polarization.” Leonard rhetorically asks Sheldon, “So, the whole scientific community is supposed to just take your word?” Sheldon confidently (but somewhat cryptically) replies, “They’re not supposed to, but they should.”

There’s a complex undercurrent here, which will prove vital to understanding the Leonard/Leslie breakup. The equ-ations of string theory demand extra dimensions, which is sufficient for Sheldon to be confident of their existence. Leonard, as an experimentalist, scoffs that the theory doesn’t match up with our known experience (which suggests only four spacetime dimensions: up/down, left/right, front/back, and a dimension representing our movement through time). Furthermore, it is unclear whether string theory offers any testable predictions. So, is it science or not?

This brings us to the heart of the issue with string theory, the Leonard/Leslie breakup, and, really, all of theoretical science: how much trust should be put in the equations, absent experimental confirmation? Certainly, based on what we know so far, there’s no real reason to put any faith in this crazy theory, but in the 1970s there was another fascinating discovery. Physicists had applied concepts from quantum mechanics to gravity and had predicted that if these two paradigms were unified, then there would need to be a boson explaining the force of gravity. They called this particle the graviton and predicted the properties that it would need to have.

Here’s the fascinating discovery: the equations for string theory predict—in fact, demand—that boson particles with exactly these properties exist. Under string theory, gravitons have to exist and, therefore, gravity also has to exist. This is the linchpin for string theory as a theory of quantum gravity. It was created to explain particle interactions but seems to require that the universe it describes must have gravity. Thus, the prediction that string theory makes about gravitons is extremely powerful. If the math is correct, it would make great strides toward a unified theory.

Hang-Ups, Breakups, and New Beginnings

There are still a lot of hang-ups with string theory (like all those extra dimensions), but it is clearly the most well-developed and influential theory of quantum gravity we have. Yet it is not the only one. By most standards, string theory’s biggest competitor is the theory of loop quantum gravity, which is championed by Leslie Winkle in The Big Bang Theory. Loop quantum gravity is in much the same state that string theory was in the late 1970s. It’s an interesting theory, but aside from a small cabal of dedicated theorists, most scientists think the equations don’t do what is needed and see little merit in the approach.

In this theory, instead of the fundamental nature of matter and forces, scientists look at the fundamental nature of spacetime and view it in tiny quantum increments. It interacts in various “loops,” which is where the field gets its name. Consequently, Leslie operates out of the mainstream, as illustrated by Sheldon’s frequent attacks against her, explicitly in “The Codpiece Topology,” which focus (mostly) on the quality of her scientific work, equating it to a game of “one potato, two potato.” Moreover, he claims, “Her research methodology is sloppy, she’s unjustifiably arrogant about loop quantum gravity, and, to make matters worse, she’s often mean to me.” For all of that, it’s certainly easy for a viewer to believe that Sheldon’s evaluation is driven more by emotion than by objectivity, which might be another key to better appreciating their professional clashes.8

To better understand how two seemingly intelligent scientists can be so certain in their opposing ways, let’s consider what they’re really doing. They’re working with equations that purport to describe reality but that contain other bits of information, other parameters, which theoretical physicists have devised in a “what if” scenario. Within the current stage of our universe, at the sorts of energy levels we normally interact with, the two theories match up with our observations and also with general relativity and quantum physics.

It’s the extra parameters, the exotic situations, that make things interesting. These can be used to predict how black holes behave, what the universe was like close to its beginning, and what sorts of results to expect in the high energies of the Large Hadron Collider. For most cases, these parameters have no impact on what we expect to see; it’s only in the hypothetical world of theoretical extrapolation that one theory has an upper hand over the others.

This brings us back to the scene at the beginning of this chapter: the Winkle/Hofstadter breakup. It all focuses around the roles of string theory and loop quantum gravity—neither of which has any discernable experimental support—as the proper approach to unifying physics under a more comprehensive paradigm. Sheldon obviously prefers string theory. As is Sheldon, Leslie is motivated by a strong commitment to the equations and their consequences, but she is committed to a different set of equations. She is opposed to Sheldon’s equations, but she plays every bit the part of a theoretical physicist as he does. Leonard is still holding out for more experimental proof and implicitly doubts Leslie’s conclusions. Worse, he seems a bit sympathetic to string theory. This is too much for Leslie, signaling the “deal breaker.” Yet might this be hubris on Leslie’s part? After all, Sheldon sometimes allows nonrational factors to creep into his professional work. (How might we test for that?) In any event, Leonard is an experimental scientist, embracing Feynman’s skeptical approach to science much more strongly than either Leslie or Sheldon does. He seeks not merely internal consistency of the mathematical equations; he wants to discover how the universe works by confirming those equations.9 Can he be blamed for that?

Of course, Leonard also wants to put the necktie back on the doorknob, but revisiting that bit of semiotics will have to wait a few more episodes. Leslie has left the building. Although Leonard and Leslie seem to be ended—for good—it is my hope that this chapter spurs you to learn more about what science is and what scientists do (even if no one calls you a “magnificent beast” for doing so).

NOTES

1. The term scientific paradigm is usually attributed to Thomas Kuhn. See his The Structure of Scientific Revolutions, 2nd ed. (Chicago: University of Chicago Press, 1970).

2. Ibid., 5.

3. Ibid., 77.

4. Ibid., 42.

5. Lawrence Krauss, Quantum Man: Richard Feynman’s Life in Science (New York: W. W. Norton & Company, 2011), quoting Richard P. Feynman, 1.

6. Richard P. Feynman, “What Is Science?” The Physics Teacher (September 1969).

7. Based on his previous theoretical work, Dirac proposed the idea of antimatter particles in 1930. These particles are exactly like ordinary matter but with an opposite charge. In 1932, Carl David Anderson experimentally discovered the positron in cosmic rays in 1932. Dirac received the 1933 Nobel Prize in Physics (jointly with Erwin Schrodinger) “for the discovery of new productive forms of atomic theory,” while in 1936 Anderson received it “for his discovery of the positron. For more on these scientists and their accomplishments, see http://nobelprize.org/nobel_prizes/physics/laureates/1933/ and http://nobelprize.org/nobel_prizes/physics/laureates/1936/, respectively.

8. Nothing about Leslie suggests she is an inept scientist; in “The Cushion Saturation,” we learn that her research is worthy enough to justify a trip to the CERN Large Hadron Collider in Geneva, Switzerland. Furthermore, unlike Sheldon, Leslie seems equally comfortable in both the experimental and the theoretical realms. When she’s introduced, it’s said that she works in the same lab as Leonard, and in her first appearance she is trying to use a laser to heat up a cup of noodles.

9. Due to the lack of discernable experiential evidence, it is tempting to interpret the debate between Sheldon and Leslie as if they were fundamentalists of different religions. This might explain why Leslie refers to “the children”; although the analogy is a bit loaded, many parents face the difficult issue of deciding in which religion their children should be raised. Leonard suggests a nonpartisan approach of allowing the children to choose for themselves, but Leslie suggests that they need more guidance. In any event, the analogy breaks down in the sense that if experimental evidence ever firmly contradicted string theory or loop quantum gravity, one would expect Sheldon or Leslie to eventually concede the point, which isn’t typically a factor in religious partisanship.